In recent years, in hygiene products such as disposable diapers, sanitary napkins and incontinent pads, the water-absorbing resin as a composition material thereof has been widely used, in view of absorbing body liquid. As such a water-absorbing resin, for example, there have been known a cross-linked product of partially neutralized polyacrylic acid, a hydrolysate of a starch-acrylic acid graft polymer, a saponified product of a vinyl acetate-acrylate ester copolymer, a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer and a cross-linked product thereof, and a cross-linked product of a cationic monomer and the like. This water-absorbing resin may be used after converted to also a sheet-like, fiber-like or film-like form, however, it is generally used after converted to a powder-like form (particulate). As such powders (particles), for example, the particulate water-absorbing agent, having a weight average particle diameter thereof of about 200 to 800 μm, is used widely.
The particulate water-absorbing agent is produced via the drying step, the crushing step, the classification step, the surface cross-linking step, and the like. In an industrial scale production, a transportation apparatus is used for transportation between apparatuses performing each step. As a transportation system, mechanical transportation and pneumatic transportation are included. In mechanical transportation, for example, a conveyor is used as the transportation apparatus.
The pneumatic transportation has many merits as compared with the mechanical transportation. As merits of the pneumatic transportation, less mechanical troubles caused by less mechanical parts, superiority in durability, a return line not necessary different from a belt conveyer, that is, one way, less commingling of foreign materials and the like are included.
A system of pneumatic transportation is largely classified to pneumatic transportation in low concentration and pneumatic transportation in high concentration. In pneumatic transportation in low concentration, a powder substance to be transported is in a dispersed state inside a transportation pipeline. In pneumatic transportation in high concentration, a powder substance to be transported moves inside the transportation pipeline while forming a group called a slag or a plug. Because high air flow velocity is required for the powder substance to maintain a dispersed state, pneumatic transportation in low concentration is high velocity transportation. On the other hand, pneumatic transportation in high concentration is low velocity transportation.
During pneumatic transportation, collision or friction may occur in particles themselves of a substance to be transported (powder substance). In addition, in pneumatic transportation, the powder substance may collide with a pipeline. In addition, collision or friction may occur between the powder substance and the pipeline. The powder substance may be worn or broken during pneumatic transportation. Pneumatic transportation may give influence on property of the water-absorbing resin powder substance. Therefore, pneumatic transportation which is capable of suppressing property decrease of the powder substance is required.
In JP-A-2004-345804, a transportation method which is capable of suppressing property decrease has been disclosed for the water-absorbing resin powder substance containing a polyhydric alcohol. In Patent Documents 2 to 4, a pneumatic transportation method for the water-absorbing resin has been disclosed where property decrease during transportation is low. In order to attain such an object, Patent Document 2 has specified curvature radius of a pipeline to equal to or higher than 5 times pipeline diameter, and Patent Document 3 has specified gas speed and solid-gas ratio, as well as Patent Document 4 has specified Froude number. Still more, Patent Document 5 has disclosed an aggregation prevention method for the water-absorbing resin during transportation, by heating or retaining heat of an apparatus, and has disclosed pneumatic transportation as an example of a transportation method.